Method and system for computer network link with undefined termination condition

ABSTRACT

The location of a termination in a properly terminated LAN can be remotely detected. The cable&#39;s skin effect produces a detectable signature at the sending-end when a step function, for example, reaches the termination. Accordingly, a network analysis device is connected to the network to inject the step function onto the network cabling. The voltage response of the cabling to this is first digitally sampled and then analyzed in a system controller. The system controller reviews the sampled data for an inflection point and then locates the termination by reference to the delay between when the signal was placed on the cable and the detection of the inflection point.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.09/401,674, filed Sep. 22, 1999, now issued U.S. Pat. No. 6,324,168, andclaims priority to U.S. application Ser. No. 08/890,486, filed Jul. 9,1997, now issued U.S. Pat. No. 6,016,404 and U.S. ProvisionalApplication Ser. Nos. 60/021,487, filed Jul. 10, 1996, and 60/029,046,filed Oct. 29, 1996, the entire teachings of these applications beingincorporated herein by this reference.

BACKGROUND OF THE INVENTION

Physically, local area networks (LAN) comprise a transmission medium andnetwork devices that transmit through it. The transmission medium istypically coaxial or twisted-pair wiring. The network devices or nodesare the network cards of computer workstations that utilize the networkcabling to communicate with each other. Dedicated network devices suchas hubs, repeaters, bridges, switches, and routers are also used tomanage or extend a given LAN or act as inter-networking devices.

One of the most common protocols for a LAN is termed carrier sensemultiple access with collision detection (CSMA/CD). This protocol issometimes generically, but incorrectly, referred to as Ethernet, whichis a product of the XEROX Corporation. I.E.E.E. has promulgatedstandards for this protocol; IEEE 802.3 covers 1-persistent CSMA/CDaccess method and physical layer specification. The protocol comes invarious implementations, 10Base(2) and (5) are 10 megabit per second(MBPS) networks using different gauges of coaxial cable (2 and 5) in abus topology. 10Base(T) also operates at 10 MBPS but uses twisted-paircabling in a star topology in which each node connects to a hub. Newer100 MBPS implementations such as 100Base(T) are becoming more commonwith 1 GigaBPS devices in planning and testing.

A number of problems can arise at a LAN's physical layer. In the case oftwisted-pair or coaxial wiring, the electrical conductors may becomefrayed or broken The shielding may be damaged, failing to protect theconductors from surrounding electromagnetic interference and changingthe cable's characteristic impedance. Moreover, the terminators at theend of the network cables in bus topologies or the terminators in thenodes at the ends of the links in star topologies may be poorly matchedto the characteristic impedance of the network's cables or non-existent.This produces signal reflections that can impair the operation of thenetwork.

Another potential problem with a network is the fact that cabling may betoo long. The IEEE 802.3 10Base(T) protocol, for example, limits thecable length to 200 meters with repeaters. This restriction is placed onnetworks because signals require a non-trivial time to propagate throughthe entire length of a CSMA/CD network relative to the data rate of thenetwork. Network devices, however, must have some assurance that afterthey have been transmitting for some minimum time that a collision willnot thereafter occur. Additionally, the end of each packet transmissionmust be allowed to propagate across the entire network before the nexttransmission may take place. If the cabling is long, the time allocatedto this may begin to consume too much of the network's potentialbandwidth.

A number of techniques exist for validating a network at the physicallayer. The most common approach is called time domain reflectometry(TDR). According to this technique, a predetermined signal, typically astep-function, is injected onto the network cabling. The TDR system willthen listen for any returning echoes. Echoes arise from the signalpassing through regions of the cable where the characteristic impedancechanges. Based upon the amplitude of these reflections and the delaysbetween the transmission of the signal onto the cable at the sending-endand the receipt of the reflection back at the sending-end, the locationof the impedance change, a frayed portion of cable for example, may belocated.

TDR has been used to determine the length of the network cable and thuswhether it conforms to the relevant protocol. Prior to testing, thenetwork's terminators are removed and the conductors are shortedtogether or open circuited. The length of the cable may then bedetermined based on the time delay between when the TDR signal is placedon the network and when the open- or short-circuit reflection isdetected at the sending-end.

SUMMARY OF THE INVENTION

The problem with known cable length detection methods is that they relyon the removal of the terminators or on a reflection producing device.The terminators, however, are necessary to the proper operation of thenetwork. Thus, the cable length can only be determined on anon-operational network. This requirement is not unduly restrictive inthe case of validating a newly installed network since the TDR analysismay be performed prior to the installation of the terminators orattachment of the nodes. This requirement, however, negates the periodicmonitoring of an operational network and the diagnosis of a previouslyinstalled network that is exhibiting problems.

In the invention, the location of a properly configured terminator,i.e., a terminator that has been configured to closely match the nominalcharacteristic impedance of the network cabling, can be remotelydetected by analyzing the network's response to a predetermined signalfor skin effects. In more detail, the terminator produces a signaturethat is detectable at the sending-end when predetermined signal, such asa current step function, is injected onto the network cabling. Themagnitude of the voltage at the sending-end will slowly increase. Thisresults from the skin effects and accumulated d.c. resistance across thelength of the cable as the step function propagates down its length.After a time corresponding to twice the propagation time between thesending-end and the terminator, the voltage will undergo an inflection.After this inflection, the voltage asymptotically returns to the voltagelevel initially produced when the step function was generated.

In general, according to one aspect, the invention is directed to amethod for analyzing a network link on a computer network. Specifically,it analyzes the link under any one of three criteria. Specifically, ashort circuit threshold is applied to the link's response, an opencircuit threshold is applied to the response, and a search is performedfor a matched termination. A decision is then made based upon theapplication of these thresholds and the matched terminator search. Then,once the type of terminator and its is found, a determination of thetime delay between the generation of the predetermined signal and thelocated termination is performed.

In general, according to one aspect, the invention is directed to amethod for analyzing a network link on a computer network. Specifically,it analyzes the link under any one of three criteria. Specifically, ashort circuit threshold is applied to the links response, an opencircuit threshold is applied to the response, and a search is performedfor a matched termination. A decision is then made based upon theapplication of these thresholds and the matched terminator search. Then,once the type of termination is found, a determination of the time delaybetween the generation of the predetermined signal and the locatedtermination is performed.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1A is a block diagram showing a network diagnostic device of thepresent invention;

FIG. 1B is a schematic diagram showing the cross-connect panelsupporting physical layer access for the MIU according to the presentinvention;

FIG. 2 is a timing diagram showing a hybrid packet/step function fordetecting terminations on an active network;

FIG. 3 is a state diagram illustrating the operation of the packet/stepfunction generator;

FIG. 4 is a graph of the voltage as function of time when a stepfunction is placed on the network cabling of a LAN;

FIG. 5 is a process diagram illustrating the data processing performedon the cable's response to the TDR edge to find the terminator;

FIG. 6 is a voltage vs. time plot of an exemplary cable response;

FIG. 7 shows a low pass filtered and first order differential of theexemplary cable's response;

FIG. 8 shows another exemplary cable response illustrating a correctionfor the real resistance of the cable;

FIG. 9 is an impedance spectrum for the cable response shown in FIG. 8;

FIG. 10 is another exemplary cable response showing the effect of an LCcircuit;

FIG. 11 is an impedance spectrum for the cable response of FIG. 10;

FIG. 12 is a schematic diagram showing the elements of a network linkand the point of attachment of the MIU;

FIG. 13 is a process diagram showing the isolation of the node-sideresponse of the link; and

FIG. 14 is a graphical user interface displaying the information gainedfrom the, terminator detection according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A illustrates a network diagnostic device 50 implementing thepresent invention. Other aspects of the device are described in U.S.patent application Ser. No. 08/619,934, filed on Mar. 18, 1996, entitledPacket Network Monitoring Device, the teachings of which areincorporated herein in their entirety by this reference.

The illustrated network 5 is configured in a star topology, such as in10Base(T). It incorporates multiple links 10–15, which operate in acommon collision domain, although separate collision domains could existbetween the links in other implementations. The nodes or computers 16–21are located at the ends of network cables 22–27 for each of the links.Each of the nodes includes a terminator 28–33 that is matched to thecharacteristic impedance of the corresponding cables. In the case wereeach of the links 10–15 is a bus-style network connecting severalcomputers, separate terminator devices are connected at the ends of thenetwork cables 22–27. A hub 16, or alternatively switch or other networkcommunications device, enables communications between the nodes byretransmitting packets between the links.

A media interface unit (MIU) 100, or attachment unit, connects adigitizer 120 and signal generation circuits 150 to the physical layerof the network's links 10–15 between terminating hub 16 and nodes 16–21,which include terminators 28–33. The MIU includes the receiver units Rthat collectively provide a two-channel input to the digitizer 120through a summing network 36. The summing network 36 enables individuallinks to be monitored, or combines the signals of multiple links, on achannel of the digitizer 120. For adequate analog resolution, thedigitizer should have at least a 500 MM sampling frequency with eightbits of resolution per sample and a long memory capacity of at least onemegabyte of eight bit samples, or preferably 2 to 4 megabytes for 10MBPS networks. Analysis of 100 MBPS to 1 GBPS networks is facilitatedwith correspondingly faster sampling frequencies and longer memorycapacities.

FIG. 1B shows one implementation of the MIU 100 integrated into a crossconnect panel of the invention. Each remote network node 16–21 isconnected to a wall panel W1–W6, commonly located in the office 68 inwhich the computer 20 is located. These wall panels receive, in oneimplementation, four twisted pair wires supporting a communications linkin a common jack or connector scheme. The wires 70 from the wall panelsW1–W6 are bundled into larger horizontal cables 60 of 24 to 48 separategroups of four twisted-pair wires from other nodes in other offices.

Each of the horizontal cables 14 terminates usually in a wiring closet72 housing the cross-connect panel 64 and the network communicationsdevice 16. Each group of wires of a communications link is associatedwith and electrically connected to a separate panel-device connector 66on the front of the cross-connect panel 10. Short jumper cables or patchcords 62 are used between each panel-device connector 66 of thecross-connect panel 64 and the network device 16.

Generally, cross-connect panels provide a convenient way to terminatethe horizontal cables 60 while allowing computers to be connected todifferent ports of the network communication device 16. Moreover, thenetwork communications device may be replaced simply by switching thepatch cords 24.

The panel 64 includes a monitoring port to which the media interfaceunit (MIU) 100 is connectable. The port provides physical layer accessto the communications links by supporting direct signal taps to thecommunications media of the links.

Returning to FIG. 1A, the digitizer 120 comprises a buffering amplifier122 a, 122 b on each of the two channels Ch1, Ch2. Two sample-and-holdcircuits 124 a, 124 b downstream of each amplifier freeze the detectedvoltage in each channel for digitization by two analog-to-digitalconverters 126 a, 126 b. The digital outputs of the converters arewritten into two long memories 128 a, 128 b, one assigned to eachchannel Ch1, Ch2. The memories 128 a, 128 b

A system processor 140 is connected to read the arrays of data from thelong memories 128 a, 128 b of the digitizer 120. In one implementation,it is a personal computer/workstation, which is also connected to thenetwork 5 via a conventional network card. The system processor 140performs signal processing on the data arrays. The system processor 140also provides the overall control of the device 50.

A packet/step function signal generator 150, also under the control ofthe system processor 140, is connected to the network 5 via drivecircuits D. The signal generator 150 has much of the control logic thatwould be contained in a network card for the relevant network. It candetermine when other nodes are transmitting, determine incidences ofcollisions, and assess when a packet transmission can be made inaccordance with the network's protocol.

The signal generator 150 produces a hybrid step/packet transmission inorder to allow the device 50 to perform terminator and physical layeranalysis while the network 5 is operational. Nodes 16–21 can behaveunpredictably if a lone step function is transmitted over an idlenetwork. The nodes, however, will generally ignore a packet transmissionas long as it is not addressed to the nodes. To utilize this behavior,the signal generator 150 is configured to generate a broadcastdiagnostic packet. Packets with this source and destination address willbe universally ignored by the network's nodes. The step function isinserted where a data payload would typically be found. This rendersstep function transparent to the nodes.

FIG. 2 schematically shows the hybrid step/packet transmission 200 for a10Base(T) CSMA/CD network. In compliance with the network's protocol,the packet 200 has a standard length preamble 210. The source anddestination addresses 220, 230 conform to a diagnostic broadcast packet.A data payload 240 is started, but then after a predetermined time, thevoltage on the cabling is held at a quiescent level, i.e. 0 Volts inmost networks, for time t₁. This period corresponds to the time that isrequired for a signal to traverse the entire network, usually between 1and 6 microseconds. This delay allows any echoes to die out. Then, theedge 250 of the current step function 252 is generated by producing apredetermined amount of current on the network cabling. This raises themagnitude of the voltage on the cabling to a selected level, e.g. 0.5–5Volts. As shown, this voltage is preferably close to the normal voltageswings experienced during data transmission, but a strongersignal-to-noise ratio can be obtained by using higher voltages. Ineither case, the voltage swing should not be so large as to create therisk of damage to any of the node's network cards.

The step function 252 is maintained for a time that is long enough toallow the edge to propagate throughout the network and the responsereceived back by the digitizer at the sending-end at time t₂. At theexpiration of this time, the voltage on the network is brought back to aquiescent state allowing the other nodes on the network to recognize theend of the transmission.

The digitizer 120 is used to detect the response of the network 10 tothe step function. A trigger device 130 of the digitizer is armed by thesystem processor 140 and triggered by the packet/step generator 150 inresponse to the transmission of the hybrid packet on the network. Thesystem processor then extracts any detectable echo from the sampledevent. By analyzing the echo, the location of network termination isdetermined.

FIG. 3 is a state diagram for the packet/step function generator 150.The generator is activated by a transmit command that is received fromthe system controller 140 in step 290. It then prepares to send thepacket in step 291. First, it waits until the network is idle in step292. When there are no transmissions on the network cabling, an externaltrigger is sent to the trigger device 130 of the digitizer 120 in step293 and the packet 200 is transmitted on the network in step 294. Thegenerator then waits until the packet transmission is finished in step295. This transmission includes the broadcast source and destinationaddress 230,220, the start of the payload 240, and the “silent” time t₁to allow echoes to die out. The edge 250 is then sent in step 296. Itagain waits for the conclusion after time t₂ and then signals the systemcontroller 140 in steps 297 and 298.

FIG. 4 shows an exemplary network response to the current step function252 at the sending-end. For clarity, only the portion of sampled signalthat resulted from to the step function is shown. In the properlyterminated network, the response of the network to the step function 252exhibits two slopes. The first slope 410 is indicative of theaccumulated DC resistance and skin effects of the cable as the edge 250propagates towards the terminations 28–33. The magnitude of the voltageslowly increases from its initial level v1 just after the edge 250. At atime that corresponds to twice the propagation time T_(a) between theinsertion point for the edge 250 and the termination, the networkresponse exhibits an inflection point 412 where the voltage begins todrift back to its initial level v1 with a new slope 414.

The system processor 140 processes this data from the digitizer 120 andlocates the inflection point 412 and determines time T_(a). Then, byreference to the signal propagation speeds across the network cabling22–27 the processor 140 calculates the distance to the terminators28–33.

FIG. 5 illustrates the signal processing performed by the systemprocessor 140. The first step in the signal processing is to isolate thecable's response to the TDR edge in step 510. As described in connectionwith FIG. 3, the data acquisition is triggered in response to thebeginning of the hybrid/TDR packet shown in FIG. 2. For the analysis,however, the only relevant portion of the sampled signal event is thecable's response to the TDR edge 250. FIG. 4 shows an isolated cableresponse. This response is exceptionally clean, without distortion,example where the terminator inflection point 412 is very evident.

Possibly a more common cable response, or worse case situation, is shownin FIG. 6. This cable response shows a number of changes in the cable'scharacteristic impedance 610 at various time delays from the TDR edge250. These could be caused by cable splices, different types of cable,or damage to the cable shielding, for example.

The data that corresponds to the cable's response is then low passfiltered in step 520. This filtering smooths any high frequency spikesin the cable's response to facilitate the analysis of the trends in thedata. The response labeled 710 in FIG. 7 shows the low pass filteredresponse of the cable response shown in FIG. 6. Many of the spikes 610are removed in the low pass filtered response 710 of FIG. 7.

The filtered data is then compared to short and open circuit thresholdsin step 522 of FIG. 5. If the end of the circuit is not properlyterminated in the characteristic impedance, but is an open circuit, thecable's response will be characterized by an increase in the voltage ata delay that corresponds to the distance to the open circuit. Theinductance associated with the cable length causes the detected responseto increase to twice the voltage of the initial TDR edge 250 in the caseof a complete open circuit. An appropriate open-circuit threshold is190% of the voltage of the TDR edge. This threshold is applied to thecable's response to determine whether an open circuit exists and thedistance to the open circuit represented by the time delay at which thethreshold is exceeded.

Contrastingly, a short circuit at the end of the cable will becharacterized by drop in the magnitude of the voltage on the cable. Thiscorresponds to the dissipation of the energy of the TDR edge 250 toground as the step function reaches the short-circuit. An appropriateshort-circuit threshold is 10% of the voltage of the step function'smagnitude. This threshold is similarly applied to the cable's responsein step 522.

If either of the open or short circuit thresholds are exceeded asdetermined in step 524, the distance to the short or open circuitcorresponds to one-half the delay until either of thresholds areexceeded in the cable's response in step 526.

On the assumption that a short or open-circuit is not detected, i.e.,the cable is probably properly terminated with a termination thatclosely matches the cable's characteristic impedance, a first-orderdifferential is performed on the cable's response to the TDR edge 250 instep 528. The result of this processing is a plot showing how thecable's response is changing as a function of the delay from the TDRedge. The first order differential of the low-pass filtered response 710is identified as 712 also in FIG. 7.

The terminator is then located by finding the highest time delay atwhich the first order differential becomes positive and remains positivein step 530. This analysis is conceptionalized by beginning at the rightside of the first-order differential plot 712 in FIG. 7, and thenscanning leftward until the zero-crossing is found (see reference 714).This corresponds to the inflection point 412 described in connectionwith FIG. 4, and the time delay between the zero-crossing 714 and theTDR edge 250 corresponds to twice the distance to the terminator or nodeat the end of the cable. The physical distance is calculated bymultiplying the time delay by the propagation speed of the signals overthe cable and dividing by two.

As shown in steps 532 and 534, further processing is optionallyperformed to identify impedance problems with the network cabling.First, a resistance correction must be performed in factor out thecontribution of the real portion of the cable's resistance to theresponse in step 532. Since the real resistance has no frequencydependence, by definition, it will not distort any signals transmittedon the line other than causing attenuation.

FIG. 8 shows an exemplary cable impedance as a function of time delay.Plot 810 is the total, real and imaginary, impedance as a function oftime delay. Plot 812 represents the response that is dictated by onlythe imaginary portion, i.e., reactance, of the cable's impedance. Animpedance frequency spectrum shown in FIG. 9 is then generated bynormalization and by performing a fast Fourier transform (FFT) on theimpedance response 812. This analysis exposes problems with the cablethat may cause improper operation in certain networks by showing thefrequencies that are subject to distortion.

For example, the cable response shown in FIG. 10 could occur in the caseof a LC circuit that creates the ringing effect shown by the briefsinusoidal pulse 1010. The frequency analysis of this response is shownin FIG. 11. The spike at 35 MHZ is characteristic of the LC circuit ringshown in FIG. 10. By this analysis, the distortion caused by the LCcircuit would not be a problem in a 10Base(T) network, for example. Thatnetwork is band limited under 35 MHZ. This spike, however, could presentsubstantial problems to a 100Base(T) network whose operation frequencyrange includes 35 MHZ.

In some instances, added processing or compensation is necessary wherethe injection point of the TDR step function signal is not at or near anend of the network link. FIG. 12 is a block diagram showing thecomponents forming a monitored link 20 when the cross-connect or patchpanel 64 of FIG. 1B is used to connect the network diagnostic device 50to the network 5. The patch panel 64 supporting the connection to thenetwork link's physical layer is located at a non-end point on the link.A patch cable 62 connects the panel 64 to the hub or other networkdevice 16 usually via a standardized connector 74, an RJ-45-typeconnector, in the illustrated implementation. The other end of the link14 extends from a punch-down channel 76 in the panel 64 through thehorizontal cable 60 to a wall box W1 at an office, which has a secondpunch-down channel 78 and connector 80. A second patch cable 82typically connects the computer or network device 20 to the wall box W1.

The TDR signal injected at the patch panel propagates both ways alongthe link 14 from the point of injection 84. It travels to the hub 16 andto the node 20. Thus the response detected at the panel 64 is acomposite response of the connection to the hub-side 86 and node-side 88of the link 14. This effect undermines the previously describedanalysis.

FIG. 13 is a process diagram showing calibration processing typicallyperformed when the network is first installed to isolate the node-sideresponse. The node-side 88 of the patch panel 64 is opened or closedcircuited at the punch down channel 76 in step 1310. The TDR signal isthen injected onto the cabling in step 1320. The detected response isonly that of hub-side 86 of the link 14. This response is stored inconnection with the link 14 in step 1340.

The stored hub-side response is later used to analyze the node-side 88of the link 14. The response of the link to the TDR signal is detectedin the fully connected and functioning network. This response is acomposite of the node-side and the hub-side responses. The hub-sideresponse determined during calibrating, however, is subtracted from thecomposite response to isolate the node-side response. This node-sideportion of the link is typically the most important for monitoringpurposes since it is more susceptible to acquired damage andunauthorized changes.

FIG. 14 is a user interface that displays the information gained fromthe TDR cable length analysis on a monitor 142 of the system processor140. For a 36 port hub, TDR analysis can be separately performed on eachlink. In the two-channel device shown in FIG. 1A, this probing mustoccur serially. The cable length derived from the analysis can then bedisplayed as shown also noting the maximum allowable cable length forthe protocol and media type, here shown as 100 meters. Those links thatdo not conform with the protocol can be displayed as exceeding thatdistance. Additionally, the status of each of the links can be assessedand displayed. By reference to the impedance spectrum and thecharacteristics of the termination, open or short circuit, for example,problems can also be indicated by graphically identifying that terminalsor links that require maintenance and the users on the is.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A method for analyzing a network link in a computer network,comprising: generating a predetermined signal on the network link;detecting a response of the link to the predetermined signal; analyzingthe response for an influence of a termination of the link, in which thestep of analyzing comprises: applying a short circuit threshold to theresponse of the link, applying an open circuit threshold to the responseof the link, and searching the response of the link for a matchedterminator; locating the termination of the link in response to theapplication of the short circuit threshold, open circuit threshold, andsearch for the matched terminator; and determining a time delay betweenthe generation of the predetermined signal and the located termination.2. The method described in claim 1, wherein the step of searching forthe response of the matched terminator comprises determining a change inthe influence of skin effects on the response resulting from thepredetermined signal reaching the terminator.
 3. The method described inclaim 1, wherein generating the predetermined signal comprisesgenerating a current step function on the network link.
 4. The methoddescribed in claim 1, wherein the step of searching for the response ofthe matched terminator comprises detecting an inflection point ininduced voltage on the network link.
 5. The method described in claim 1,further comprising calculating the length of the network link to theterminator in response to the time delay.
 6. The method described inclaim 1, wherein the step of analyzing further comprises low passfiltering the response of the link and then detecting an inflectionpoint in filtered response of the network link.
 7. The method describedin claim 1, wherein the step of analyzing further comprises low passfiltering the response of the link and then applying the thresholds tothe filtered response.
 8. The method described in claim 1, wherein thedetection of the response of the link to the predetermined signal occursat a non-terminator location on the network link.
 9. The methoddescribed in claim 1, wherein the generation of the predetermined signalon the network link occurs at a non-terminator location on the networklink.
 10. The method described in claim 1, wherein the detection of theresponse of the link to the predetermined signal occurs on anoperational computer network.
 11. The method described in claim 1,wherein the generation of the predetermined signal on the network linkoccurs on an operational computer network.
 12. A network terminationanalysis device for a digital data network, comprising: a functiongenerator that injects a predetermined signal onto cabling of thenetwork; a digitizer that digitally samples the network's response tothe predetermined signal; and a system processor that downloads datafrom the digitizer to analyze the network's response to thepredetermined signal and identify a time between the generation of thepredetermined signal and a change in the network's response due to atermination of the network, in which the analysis comprises applying ashort circuit threshold to the response, applying an open circuitthreshold to the response, and searching the response for a matchedterminator.
 13. A device as described in claim 12, wherein the functiongenerator injects a step function.
 14. The device described in claim 12,wherein the system processor calculates at least one length of thecabling based on the time between the generation of the predeterminedsignal and a change in the network's response exceeding any one of theshort or open circuit thresholds or the detection of the matchedterminator.
 15. The device described in claim 14, further comprising amonitor for displaying at least one calculated length of the cabling.16. The device described in claim 15, wherein the display furtherindicates a maximum protocol-determined length for the cabling.
 17. Thedevice described in claim 12, wherein the function generator injects thepredetermined signal onto cabling of the network at a non-terminatorlocation.
 18. The device described in claim 12, wherein the digitizersamples the network's response to the predetermined signal at anon-terminator location on the cabling.
 19. The device described inclaim 12, wherein the function generator injects the predeterminedsignal onto cabling of an operational network.
 20. The device describedin claim 12, wherein the digitizer samples the response to thepredetermined signal of an operational network.
 21. A method foranalyzing a network link in a computer network, comprising: generating apredetermined signal on the network link; detecting a response of thelink to the predetermined signal, said response including a resistanceresponse; filtering the response of the link by removing a contributionof a real component of the resistance response of the link; anddisplaying the filtered data to assist in the identification ofimpedance problems on the link.
 22. The method described in claim 21,wherein the detection of the response of the link to the predeterminedsignal occurs at a non-terminator location on the network link.
 23. Themethod described in claim 21, wherein the generation of thepredetermined signal on the network link occurs at a non-terminatorlocation on the network link.
 24. The method described in claim 21,wherein the detection of the response of the link to the predeterminedsignal occurs on an operational computer network.
 25. The methoddescribed in claim 21, wherein the generation of the predeterminedsignal on the network link occurs on an operational computer network.